This invention relates to electric illumination, and more specifically to a fluorescent lighting unit capable of providing an even, diffused lighting source. It has particular utility as a backlighting unit for twisted nematic liquid crystal displays.
Flat panel displays are used in the avionics field as a substitute for cathode ray tube (CRT) displays. In addition to reduced bulk, such displays are capable of withstanding increased mechanical stresses and require less power to operate. They further avoid the use of electron and diflection circuits common to CRTs. One type of flat panel display utilizes twisted nematic liquid crystals. More often provided without backlighting, such displays can be illuminated impaired. Such displays may be either in full color or monochromatic. It may also be desirable to operate such displays in a manner which is compatible with night vision goggles, wherein light from the display does not adversely interfere with the operation of the night vision goggles. The illumination level of the display is sometimes adjusted in order to compensate for differences in ambient lighting conditions. For example, during nighttime operations, it is desired that the lighting level of an aircraft cockpit or similar environment not greatly exceed that of the ambient environment, as viewed by the crew member.
One difficulty in the operation of fluorescent lighting devices is that flicker tends to occur during conditions of low illumination, such as in cold ambient conditions or reduced operating voltage when the lighting circuit is dimmed.
Accordingly, it is desired that to provide an illumination source which has an even light distribution and whose light output can be readily controlled. It may be desired that the light output be provided in particular colors which cooperate with twisted nematic liquid crystal substances to provide a desired color output or to reduce interference with night vision goggles. The light source should also be durable and capable of withstanding stresses such as those anticipated in the military cockpit environment.